Ok lets back it up and start with the basics. Power is not measured in volts. It is measured in watts. We get the number of watts that a motor will output by multiplying the voltage by the current going through it. This can be tricky to get a measured value for so the general rule of thumb that I and several others use when it comes to electric motors is to only run them at 1.5 times their rated voltage. So in theory an 18v motor could be run at 27volts in a robot and should have a reasonable life.

Do a bit of background reading into GCSE or standard grade electrical circuit physics and you should get most of the answers you need (that's right, that stuff was actually useful!)

And with your second question, 2 12v batteries connected in series will give you 24v

http://electronics.howstuffworks.com/ev ... ttery6.htm